This invention is concerned with a system for determining time duration of angular rotation and, more particularly, with a system for determining the time duration per unit amount of angular rotation given any known number of degrees of rotation.
There are a number of applications in which it is required to know the amount of time duration required for a rotating member to rotate through a given number of degrees. In particular, in a spark type internal combustion engine it is required to know when to fire the spark plug relative to some point in the rotation of the engine crankshaft. This firing point is termed spark advance or spark retard. The amount of advance or retard is determined by, among other things, how fast the engine crankshaft is rotating.
A prior art apparatus for determining engine speed is described in U.S. Pat. No. 4,375,209 issued Mar. 1, 1983 to A. D. Robbi, et al. and assigned to the common assignee. In the patented system, a rotating element similar to a distributor and coupled to the engine crankshaft produces start and stop pulses marking the beginning and ending of a fixed number of degrees of rotation. A clock produces pulses at a known fixed rate which are counted in a counter between the times of the start and stop pulses. The count in the counter, upon receipt of the stop pulse, represents the time for the total number of degrees of rotation. Based on this information, a number is chosen from a read only memory which, utilizing a second clock pulse source, effectively reduces the count in the counter to zero at a time which provides the proper amount of spark advance or retard.